Meaning: 1. (Mass noun) Betrayal, treachery, intentional breach of faith or violation of trust. 2. (Count noun) One instance of such, as the little perfidies of organizational life.

Notes:Perfidy is basically a mass noun like contemplation and consternation that has no plural. However, it is often used to refer to a single instance of betrayal, hence perfidies becomes a possible plural form. The adjective for this noun is perfidious; it may be used adverbially with the suffix -ly. We recommend avoiding perfidy's fraternal twin, perfidity, unless you are writing a poem and need another syllable.

In Play: War, of course, is rife with perfidy: the invasion of Czechoslovakia and the attack on Pearl Harbor are two of the greatest acts of perfidy of World War II. But it is the little perfidies of life that are more likely to wear us down: "What perfidy! Lucy Morales went out with Lance Sterling last Saturday after telling me that she had to stay home to wash her hair!"

Word History: Today's Good Word comes from Latin perfidia, the noun of perfidus "treacherous". This word is made up of per-, an intensifer often indicating destruction + fides "faith". The root of these words was once Proto-Indo-European bheidh-/bhoidh- "to trust", which became Germanic *bidan "to stay, wait", found today in English abide and abode. The sound [bh] (b with a puff of air) became [f] at the beginning of a Latin word, which explains the root of fides. This word is visible in many Latin borrowings in English, including confident, affidavit, and fiduciary. After a little bit of tinkering by French, it also emerged as fiancé. (We can always trust Dr. Lew Jury to send us delicious little lexical pearls like today's, which we wrote up at his suggestion.)